AD is a progressive neurological disorder characterized by deterioration of cognitive function, dementia, memory loss, and altered behavior.
AD is the major unmet medical need in neurology, and it is estimated that, by 2050, there could be more than 100 million AD patients worldwide [Alzheimer's Association, 2013 Alzheimer's disease facts and figures. Alzheimer's & Dementia 2013, 9, 208-45]. AD dramatically affects the quality of life of the sufferers and their families, and despite massive investments, there are few, if any, effective treatments for AD.
The AD pathogenesis involves a complex interplay of genetic and biochemical factors, including an increased production of β-amyloid peptide (amyloid hypothesis) and an increased phosphorylation of the microtubule-associated tau protein (tau hypothesis) [Querfurth, H. W. and LaFerla, F. M. Mechanisms of disease: Alzheimer's disease. N Engl J Med 2010, 362, 329-44]. The β-amyloid peptide (Aβ) is generated by the amyloidogenic cleavage of the membrane-associated amyloid precursor protein. Two major enzymes have been identified as responsible for Aβ, formation, namely β-secretase (BACE-1) and γ-secretase. Huge efforts have been devoted to the identification of small molecule inhibitors of either these enzymes. γ-secretase inhibitors have reached phase III of clinical trial but they failed because of lack of efficacy and heavy side effects (i.e. skin cancer) [Blennow, K.; Zetterberg, H.; Haass, C.; Finucane, T. Semagacestat's fall: where next for AD therapies? Nat Med 2013, 19, 1214-15].
BACE-1 inhibitors are still in clinical development, with the most advanced compounds in phase III of clinical trial. BACE-1 remains one of the few target options available for a potentially efficacious treatment within the amyloid hypothesis of AD [Rafii, M. S. Update on Alzheimer's disease therapeutics. Rev Recent Clin Trials 2013, 8, 110-18].
As for the tau hypothesis, research activities have been focused on the identification of kinase inhibitors, as a few kinases have been identified as responsible for the tau phosphorylation. Among these enzymes, glycogen-synthase kinase 3β(GSK-3β) has been identified as one of the key players within this pathological cascade. GSK-3β is responsible for tau hyperphosphorylation, which causes tau to be detached from the microtubules and precipitate as intraneuronal tangle aggregates [Avila, J.; Wandosell, F.; Hernandez, F. Role of glycogen synthase kinase-3 in Alzheimer's disease pathogenesis and glycogen synthase kinase-3 inhibitors. Expert Rev Neurother 2010, 10, 703-10]. Furthermore, GSK-3β has been proposed as a possible link between β-amyloid peptide and tau protein [Hernández, F.; Gómez de Barreda, E.; Fuster-Matanzo, A.; Lucas, J. J.; Avila, J. GSK3: a possible link between beta amyloid peptide and tau protein. Exp Neurol. 2010, 223, 322-25]. Therefore, GSK-3β inhibitors have been long sought [Martinez, A.; Perez, D. I.; Gil, C. Lessons learnt from glycogen synthase kinase 3 inhibitors development for Alzheimer's disease. Curr Top Med Chem 2013; 13, 1808-19].
Despite the two hypotheses have been considered in contrast, recent evidences suggest that AD is a multifactorial disease, where several neurodegenerative pathways can concomitantly contribute to neuronal death and associated neurodegeneration [Ittner, L. M.; Götz, J. Amyloid-β and tau a toxic pas de deux in Alzheimer's disease. Nat Rev Neurosci 2011, 12, 65-72].
In this scenario, multitarget drugs (MTDs), namely small organic molecules able to hit multiple targets affecting different pathological pathways, are emerging as promising disease-modifying compounds for the treatment of complex neurological disorders [Cavalli, A.; Bolognesi, M. L.; Minarini, A.; Rosini, M.; Tumiatti, V.; Recanatini, M.; Melchiorre, C. Multi-target-directed ligands to combat neurodegenerative diseases. J Med Chem 2008, 51, 347-72. Bolognesi, M. L.; Simoni, E.; Rosini, M.; Minarini, A.; Tumiatti, V.; Melchiorre, C. Multitarget-directed ligands: innovative chemical probes and therapeutic tools against Alzheimer's disease. Curr Top Med Chem 2011, 11, 2797-806].
The strategy of targeting two or more proteins at the same time with a single compound can provide therapeutic effects superior to those of a selective drug.
This can be explained by the number of potential benefits offered by the use of MTDs over cocktails or multicomponent drugs. The advantages of MTDs can be summarized as follows: 1) reduced uncertainty in clinical development since predicting the pharmacokinetics of a single compound is much easier than with a drug cocktail, overcoming the problem of different bioavailability, pharmacokinetics and metabolism; 2) certainty on the pharmacodynamics; 3) improved efficacy due to the synergistic effect of simultaneously inhibiting multiple targets; 4) improved safety by decreasing the side effects related to the load of a drug cocktail (reduced risk of drug-drug interactions); this is particularly relevant for drug metabolism, where the competition of different drugs for the same metabolic enzyme affect their toxicity. All these considerations are of particular relevance as one of the major contributions to attrition rate in drug development continues to be the drug candidate's pharmacokinetic profiling.
Another important advantage is a simplified therapeutic regimen and improved compliance, which is particularly important for elderly AD patients and their caregivers [Small, G.; Dubois, B. A review of compliance to treatment in Alzheimer's disease: potential benefits of a transdermal patch. Curr Med Res Opin 2007, 23, 2705-2713]. With this regard, a key issue is that AD patients are susceptible to a wide range of concomitant medical conditions (comorbidity), including hypertension, vascular diseases, and diabetes, which can often be associated. Thus, problems associated with polypharmacy in the geriatric population have been recognized as critical in recent years. These problems primarily consist of drug interactions which occur more frequently in this population because of the co-existence of chronic disease and impaired organ functions. Two drugs that themselves are safe cannot be assumed to be safe in combination, particularly in elderly patients. It follows that the number of drugs administered simultaneously should be reduced as much as possible, since advanced age is an unpredictable risk factor for drug treatment [Turnheim, K. When drug therapy gets old: pharmacokinetics and pharmacodynamics in the elderly. Exp Geront 2003, 38, 843-853]. As such, MTDs are strongly favored over combination therapy with respect to the complexity of interactions between polypharmacy, comorbidity, altered pharmacodynamic sensitivity, and changes in pharmacokinetics in the elderly. The clinical use of MTDs can also simplify the therapeutic regimen [Youdim, M. B., and Buccafusco, J. J. CNS Targets for multifunctional drugs in the treatment of Alzheimer's and Parkinson's diseases. J Neural Transm 2005, 112, 519-537]. Compliance with prescribed medication regimens is essential for effective treatment. Non-compliance represents a general problem, but is especially challenging for forgetful AD patients and their caregivers [Small, G., and Dubois, B. A review of compliance to treatment in Alzheimer's disease: potential benefits of a transdermal patch. Curr Med Res Opin 2007, 23, 2705-2713]. Consequently, a simplified MTD regimen may increase treatment adherence. All the above mentioned advantages are not available with drug cocktails.
The multitarget ligand strategy is an innovative approach to the development of novel drug candidates for the treatment of complex neurological disorders, especially in view of the fact that the major basic processes involved in neurodegenerative diseases are multifactorial in nature [Cavalli, A.; Bolognesi, M. L.; Minarini, A.; Rosini, M.; Tumiatti, V.; Recanatini, M.; Melchiorre, C. Multi-target-directed ligands to combat neurodegenerative diseases. J Med Chem 2008, 51, 347-72]. Such a strategy is thus based on the concept that a single multifunctional compound can be deployed to hit multiple targets that cooperate in the neurodegenerative process underlying AD and other neurodegenerative diseases, and therefore would prevent unwanted compensation among interacting pathogenic pathways. Indeed, the multitarget compounds could represent a practical alternative to the use of drug combinations. Since most of the neurodegenerative mechanisms are shared by many neuronal disorders, such multitarget compounds may also be used as medications for other illnesses.
One problem connected to multitarget compounds is that many of them are inefficient in terms of their binding energy per unit of molecular weight. This is because they contain groups that are only important for one of the targets, being merely tolerated by the others. This results in an unbalanced profile [Morphy, R., and Rankovic, Z. Fragments, network biology and designing multiple ligands. Drug Discov Today 2007, 12, 156-160; Morphy, R. The influence of target family and functional activity on the physicochemical properties of pre-clinical compounds. J Med Chem 2006, 49, 2969-2978]. The consequent optimization of activities is not an easy task. In fact, a multitarget compound is to be considered as a new chemical entity, with its own pharmacological profile, therefore its efficacy on the targets is not predictable a priori, and the complete process of drug development must be faced.
In the context of MTDs, just to name a few, we could mention memoquin, disclosed a few years ago by Cavalli and coworkers [Cavalli, A.; Bolognesi, M. L.; Capsoni, S.; Andrisano, V.; Bartolini, M.; Margotti, E.; Cattaneo, A.; Recanatini M.; Melchiorre, C. A small molecule targeting the multifactorial nature of Alzheimer's disease. Chem Int Ed Engl 2007, 46, 3689-92], and ladostigil developed by Youdim and coworkers and currently in phase II of clinical trial [Weinreb, O.; Mandel, S.; Bar-Am, O.; Yogev-Falach, M.; Avramovich-Tirosh, Y.; Amit, T.; Youdim, M. B. Multifunctional neuroprotective derivatives of rasagiline as anti-Alzheimer's disease drug. Neurotherapeutics 2009, 6, 163-74].
WO 2008/015240 discloses a family of N-phenyl-prenylamine derivatives and claims their use to treat cognitive, neurodegenerative or neuronal diseases or disorders, such as Alzheimer's disease or Parkinson's disease; in in vitro assays, they were shown to exhibit a mild to moderate inhibitory effect on the enzymatic targets GSK-3β and only a few of them also on BACE.
EP2138488 A1 discloses 4-(pyridin-4-yl)-1H-[1,3,5]-triazin-2-one derivatives as selective GSK-3β inhibitors for the treatment of neurodegenerative diseases; in addition to be single target inhibitors, the compounds described therein are aromatic in character and display a planar geometry of the triazinone core scaffold, in which the carbon atom at position 4 of the triazinone ring that bears the pyridine ring is sp2-hybridized and is therefore unable to form stereoisomers. Consequently, the compounds of EP2138488 are achiral compounds.
In multitarget drug discovery, fragment-based approaches have been reported to play a pivotal role. In fact, fragments, rather than lead-like compounds, may have the ability to bind more than a single target [Hann, M. M.; Leach, A. R.; Harper, G. Molecular complexity and its impact on the probability of finding leads for drug discovery. J Chem Inf Comput Sci 2001, 41, 856-64. Bottegoni, G.; Favia, A. D.; Recanatini, M.; Cavalli, A. The role of fragment-based and computational methods in polypharmacology. Drug Discov Today 2012, 17, 23-34]. Then, in the fragment-to-lead step, one should carefully maintain the desired biological profile against the pathological targets, while avoiding potential liability due to off-target binding.
On these premises, a fragment-based approach has been used to design small molecules able to inhibit BACE-1 and GSK-3β enzymes, two validated targets within the two major pathological cascades of AD. Remarkably, these enzymes are ancestrally quite divergent with a sequence identity of 19%, close to the random limit. To design dual-inhibitors, a ligand-based approach has been used, combining those pharmacophoric features responsible for binding to BACE-1 and GSK-3β, such as a guanidino motif and a cyclic amide group respectively and, subsequently, docking simulations aimed at studying the interactions of the newly designed compounds into the catalytic pocket of both enzymes.
A series of 4-phenyl-6-amino-3,4-dihydro-1,3,5-triazin-2(1H)-ones turned out to be a scaffold bearing the required chemical features for binding to both targets [Prati F. et al., Structure-based design and synthesis of novel BACE-1/GSK-3β dual inhibitors. 8th Annual drug discovery conference for neurodegeneration. Feb. 2-4, 2014, Miami, Fla.].
Synthesis, identification by physical and spectral data, and associated physico-chemical properties of 4-phenyl-6-amino-3,4-dihydro-1,3,5-triazin-2(1H)-ones variously substituted at the 4-phenyl ring are described in several literature publications [Ostrogovich A., Gazzetta Chimica Italiana 1909, 39 i, 540; Ostrogovich A., Median V. B., Gazzetta Chimica Italiana 1929, 59, 181-198; Ostrogovich A., Median V. B., Gazzetta Chimica Italiana 1929, 59, 198-200; Ostrogovich A., Median V. B., Gazzetta Chimica Italiana 1934, 64, 792-800; Wakabayashi k., Okuzu M., Nippon Dojo Hiryogaku Zasshi 1970, 41(6), 237-245; Bacaloglu R. et al. Revue Roumaine de Chimie 1972, 17(4), 747-754; Neamtiu et al., Zeitschrift fuer Physikalische Chemie (Leipzig) 1976, 257, 1089-1090; Gorbatenko V. I., et al., Zhurnal Organicheskoi Khimii 1976, 12(nb.10), 2103-2107].
However, these molecules have not been considered satisfactory from the neuroprotective and neurogenesis standpoints.
There is the need in the art to find new MTDs to treat Alzheimer's disease.